(1). Field of the Invention
The present invention relates to a content measuring apparatus of a portable type, and more particularly to an apparatus for measuring contents of plant leaf which are in relation to the growth thereof.
(2). Description of the Related Art
Conventionally, the content of nitrogen has been estimated from the content of chlorophyll by such methods as making an assumption of the content of nitrogen by the comparison of the color of a leaf with a color scale, or measuring the content of chlorophyll contained in a leaf by calculating the light intensity of the red rays and the infrared rays from the light transmitted upon the light being irradiated on the leaf to be measured. Also, the fertilizer application rate has been judged so as to be in accord with the estimated content of nitrogen and the plant growing periods. Particularly, in the case of rice crop, the subsequent fertilizer application time and rate judged from the estimated nitrogen content are important points in order to ensure that the rice plant does not undergo the falling down and yet the best yield of grains is achieved.
In using the method wherein the color of the leaf is compared with the color scale, the problems are not only in the requiring of experiences but also in frequent incorrect results of judgment because the color of the leaf to the human eyes may be different depending on any weather conditions or the location and the direction of the sun light, on any differences in observation, that is, whether it is partial observation or total observation, and on the observation angle. However, because this prior art method is simple and that the color scale is inexpensive, it is true that its utilization factor is high.
A chlorophyll measuring device which substitutes for the color scale has been developed. The measuring principle of a typical chlorophyll meter may be apparent from FIG. 1 in which the leave 50 to be measured is subjected to the irradiation of light from a light source 51 and the light transmitted is measured and in which, by a dichroic mirror 52, the measurement is made of the intensity of transmitted light of the red light region (received by a light receiving diode 53) which is affected by the chlorophyll content and the intensity of the transmitted light of the infrared light region (which is received by a light receiving diode 54) which is not affected by the chlorophyll content. By calculating the light intensity difference therebetween, the estimation is made of the chlorophyll concentration per unit area without the leaf being destructed. However, in practice, the estimation of the nitrogen concentration is premised on the assumption that the chlorophyll concentration and the nitrogen concentration are on a proportional relationship, and this estimation is being utilized for the fertilizer application rate to a plant.
One of the inventors together with two others have developed and proposed in Japanese Patent Application Kokai Publication No. Hei 8-15141 a content percent measuring device for leaves of a plant with which the nitrogen content therein can be measured in a simple manner. This is explained with reference to FIG. 2. Shown in FIG. 2 is an optical measurement section 60 which is the main constituent element of the content percent measuring device for leaves of a plant, in which the light emitting means 62 for irradiating near infrared light of a predetermined wavelength onto a leaf 61 to be measured comprises a near infrared light emitting element 63 which is constituted by an element such as a light emitting diode (LED), and a narrow band filter 64 which permits the passing of only the near infrared light of a predetermined wavelength. There are provided leave holding means 66A and 66B which hold between them in planar the leaf 61 such that the near infrared light may be uniformly irradiated on the surface of the leaf 61 and which are provided with measuring windows 65A and 65B for measuring the transmitted light and the reflected light from the leaf 61. The measuring window 65B of the leave holding means 66B and the light emitting means 62 are communicated with each other by an integration sphere 67. That is, the light emitting means 62 is fixed to the integration sphere 67 so as to permit the light to be irradiated and dispersed within the integration sphere 67, and the integration sphere 67 is provided with an opening 68 which communicates with the measuring window 65B and an opening 70 at which a reflected light receiving means 69 constituted by a silicon photodiode is fixed.
As to the operation of the content percent measuring device for leaves constructed as above, the near infrared light irradiated from the light emitting means 62 is dispersed in the integration sphere 67 and is then irradiated on the surface of the leaf 61 from the measuring window 65B. Also, the light reflected from the leave 61 is dispersed in the integration sphere 67 and then is received by the reflected light receiving means 69. Further, the portion of the near infrared light that has been irradiated on the leaf 61 and transmitted therethrough is received as transmitted light by a transmitted light receiving means 71 which is constituted by a silicon photodiode fixed at the measuring window 65A side of the leave holding means 66.
In the use of the integration sphere having the above configuration, the advantages are that it is possible to increase the light intensity of the light source and that, in the case where the number of wavelengths used for measuring has become large for securing the measurement precision, the light of all the wavelengths is uniformly irradiated on the leave irrespective of the light emitting locations. However, in fabricating the integration sphere 67, its inside is given a spherical treatment, its spherical surface Is given a matte treatment and, thereafter, it is finished by gold plating so that the fabrication cost is unavoidably high.